The chemical derivatization of metal surfaces has been utilized as a means to control the interfacial reactivity of a metal in relation to processes such as adhesion, lubrication, corrosion, electrocatalysis, or electroanalysis. Understanding the factors that govern the formation of stable derivatization layers is accordingly of both technological and fundamental importance. For these reasons, monomolecular films formed from surfactant molecules possessing a head group that binds to a particular metal and a tail group that possesses a specific chemical functionality have been examined as model interfacial structures.
Thin film resonators have been, and are currently being, investigated as feedback elements in rf/microwave frequency oscillators. In general, the frequency of the resonator depends upon the mass attached to it. Thus, substrates having a thiolate monolayer thereon may find utility in thin film resonators. A thiolate is used that has an end group selected to bind selectively to a target analyte (e.g., an airborne pollutant or a solution species). When the coated resonator is exposed to an environment containing the target analyte, the analyte binds to the surface and changes the resonator frequency. The frequency change then can be correlated with the target analyte concentration in the environment. Arrays of resonators with a different end group in each element can produce a sensor for a wide range of target analytes. For example, possible applications include biomedical monitoring, industrial process monitoring, and applications for residential use (e.g.--a carbon monoxide detector coupled with a smoke detector).
Organosulfur surfactants are known to bind strongly at a variety of metals such as iron, copper, platinum, gold, and silver; and monolayer films of n-alkanethiolates on gold have been extensively studied as to their structure, electronic properties, and permeability to ion transport. In "Desorption of n-alkanethiol Monolayers from Polycrystalline Au and Ag Electrodes," J. Electroanal. Chem. Interfacial Electrochem., 1991, 310, 335-59, Widrig, Chung and Porter used voltammetric techniques to characterize monolayers formed at Au and Ag surfaces by the spontaneous adsorption of n-alkanethiols and to examine the chemistry of the bound thiol head group. Electrode reactions corresponding to the oxidative- and reductive-desorption of the adsorbed n-alkanethiolate monolayers were discussed and described. The charge found for the reductive desorption of the n-alkanethiolate monolayer at Au was consistent with the electrode reaction EQU AuSR+1e.sup.- .fwdarw.Au(O)+RS.sup.-
In the positive scan of the first voltage cycle, as described as page 345, Widrig et al. state that partial re-oxidation of the generated reduction product is apparent. It was hypothesized that the product may be either the original surface species or the corresponding dialkyl disulfide. Further, in the second cycle, it was stated that the re-oxidized material was reduced at voltages more positive than the voltage for reduction of the original surface species. When comparing voltammograms recorded at a given scan rate, the quantity of material re-oxidized, it is stated, was greater for monolayers of long-chain n-alkanethiolates because the reduced n-alkanethiolate is more likely to precipitate at the electrode surface. Reference is made to FIGS. 4B and 4C.
Widrig et al., as noted, formed the thiolate monolayer by spontaneous adsorption from ethanolic solutions. In general, the substrates were immersed in approximately 1 mM solutions of the n-alkanethiol for 2 to 24 hours, were emersed, rinsed with ethanol, and then allowed to dry.
Where control of the extent of coverage of the monomolecular layer is unimportant, self-assembly of the layer by spontaneous adsorption is generally satisfactory. Self-assembly of a monolayer in this fashion typically proceeds extremely rapidly, occurring in milliseconds or so, with many thiols. This is because the reaction is essentially diffusion controlled for many thiols, i.e., the thiol reacts as fast as it reaches the gold or other surface. For other thiols, the reaction is slower; and assembly times of minutes or hours are required.
Further, where a predetermined level of coverage is required, quite dilute solutions (&lt;10.sup.-5 M) of the thiol have been used so as to attempt to slow the reaction kinetics down enough to provide the degree of control needed for the particular application. While providing some degree of control, this dilution approach is time-consuming and laborious. Reproducibility depends upon controlling the concentration of the thiol solution. The dilute solutions used cannot usually be measured directly so the solutions are made by serial dilution. Small variations in concentrations can have a significant effect on the assembly times. Also, diffusion rates are temperature dependent, so that reproducibility and precise deposition control also require appropriate temperature control.
Further, control of the composition of multicomponent layers is also an important consideration. While it is possible to put down a partial layer of one thiolate using a dilute solution and then complete the layer with another thiolate, this approach suffers from the limitations herein discussed.
Mixed layers can also be assembled from solutions of thiol mixtures. However, the composition on the surface is not the same as the composition in the solution. Also, both thermodynamics and kinetics determine the surface composition. Satisfactory control of composition with this approach is accordingly very difficult.
Both the dilute and mixed solutions approaches also require some kind of determination of the surface composition. This can be accomplished by using, for example, X-ray photoelectron spectroscopy or electrochemistry. Such an ancillary determination requires that the end groups of the thiolates involved have different chemistries.
In part, due to the difficulties in providing a facile technique for controlling the coverage of the thiolate, the use of this technology has been limited. Accordingly, a principal object of the present invention is to provide a facile method for thiolate deposition which is characterized by enhanced control of the location and amount of thiolate deposited on a substrate such as gold.
Yet another object of the present invention lies in the provision of a method that allows formation of a mixed thiolate layer of known composition without also requiring different end groups on the thiolates being utilized.
Other objects and advantages of the present invention will become apparent as the following description proceeds.